The invention relates to a single mode optical waveguide fiber having a segmented core design which provides for high performance in the operating window around 1550 nm. In particular, effective area is large, the zero dispersion wavelength is outside the operating window, and total dispersion is positive over the operating window.
A waveguide having large effective area reduces non-linear optical effects, including self phase modulation, four wave mixing, cross phase modulation, and non-linear scattering processes, which can cause degradation of signals in high power systems. In general, a mathematical description of these non-linear effects includes the ratio, P/Aeff, where P is optical power. For example, a non-linear optical effect usually follows an equation containing a term, exp [P×Leff/Aeff], where Leff is effective length. Thus, an increase in Aeff produces a decrease in the non-linear contribution to the degradation of a light signal.
The requirement in the telecommunication industry for greater information capacity over long distances, without regenerators, has led to a reevaluation of single mode fiber index profile design.
The focus of this reevaluation has been to provide optical waveguides which:                reduce non-linear effects such as those noted above;        are optimized for the lower attenuation operating wavelength range around 1550 nm;        are compatible with optical amplifiers; and,        retain the desirable properties of optical waveguides such as high strength, fatigue resistance, and bend resistance.        
A waveguide fiber, having at least two distinct refractive index segments was found to have sufficient flexibility to meet and exceed the criteria for a high performance waveguide fiber system. The genera of segmented core designs are disclosed in detail in U.S. Pat. No. 4,715,679, Bhagavatula. Species of the profiles disclosed in the '679 patent, having properties especially suited for particular high performance telecommunications systems, are disclosed in U.S. Pat. No. 5,483,612, Gallagher et al.(the '612 patent).
The present invention is yet another core index profile species, closely related to the profiles set forth in the '612 patent, which reduces non-linear effects and which is particularly, suited to transmission of high power signals over long distances without regeneration. The definition of high power and long distance is meaningful only in the context of a particular telecommunication system, wherein a bit rate, a bit error rate, a multiplexing scheme, and perhaps optical amplifiers are specified. There are additional factors, known to those skilled in the art, which have impact upon the meaning of high power and long distance. However, for most purposes, high power is an optical power greater than about 10 mw. For example, a long distance is one in which the distance between electronic regenerators can be in excess of 100 km.
Considering the Kerr non-linearities, i.e., self phase modulation, cross phase modulation and four wave mixing, the benefit of large Aeff can be shown from the equation for refractive index. The refractive index of silica based optical waveguide fiber is known to be non-linear with respect to the light electric field. The refractive index may be written as,n=n0+n2P/Aeff,where n0 is the linear refractive index, n2 is the non-linear index coefficient, P is light power transmitted along the waveguide and Aeff is the effective area of the waveguide fiber. Because n2 is a constant of the material, increase in Aeff is essentially the only means for reducing the non-linear contribution to the refractive index, thereby reducing the impact of Kerr type non-linearities.
Thus there is a need for an optical waveguide fiber designed to have a large effective area. The window of operation of greatest interest at this time is that near 1550 nm. In addition, to further minimize four wave mixing effects, the total dispersion does not pass through zero over the range of operating wavelengths. In fact, the total dispersion remains positive over the operating window so that self phase modulation may cancel with the linear dispersion, a configuration required in soliton communication systems.